On average, a woman's peak fertility period occurs 14 days after the first day of her last period, according to the Office on Women's Health, which is part of the U.S. Department of Health and Human Services. However, not every woman has a perfectly regular menstrual cycle, which is why ClearBlue recommends using an ovulation test to identify the peak fertility period. These tests monitor a woman's hormones to find out exactly when she is at the most fertile point in her cycle.